Lightweight dual wheel casters have found considerable commercial acceptance over the last decade for supporting office furniture and equipment as well as some residential furniture. As a result of this acceptance, the manufacture of lightweight dual wheel casters has been extremely competitive and engineering efforts to reduce the cost of the casters while maintaining adequate strength, reliability and durability have been keen.
Generally the casters include a central arcuate horn member that carries a vertically arranged pintle that fits within a socket plate fixed to the underside of the furniture or equipment to be supported in a manner so that the caster has swiveling motion. This horn carries a one or two-piece axle and usually the axle is non-rotatably fixed to the horn. Each of the wheels frequently has an integral hub with a central bore that is received on the projecting ends of the axle.
There have in the past been a plurality of methods for preventing removal of the wheels after the axle has been inserted into the wheel hubs. One method has been to form an annular rib inside the wheel bore that fits within an annular recess in the axle. This arrangement has not been found adequate because it is not possible to form the rib with enough radial thickness to do an adequate job of wheel locking. This is because the wheels are plastic and the central bore is formed with a core piece and the rib is formed by an annular recess in the core, and if a single core piece is used, which it must if the wheel bore is a closed end bore, the core itself will break the rib upon axial core removal unless the rib is radially shallow. Thus this method has not been found acceptable because the wheels can be easily removed after assembly, which is undesirable for this kind of caster because it decreases caster durability and reliability.
Another method of attaching wheels to this type of dual wheel caster is shown in U.S. Pat. No. 220,938 and this uses a U-shaped metal spring clip carried by the underside of the horn that has legs that fit over radial flanges on the inner surface of the wheels around the wheel hubs. The flanges on the hub significantly increase the cost of the wheel because they cannot be molded integrally with the wheel, and the spring clip frequently becomes dislodged from the lower side of the horn permitting the wheels to freely slide off the axle.
Another method of attaching dual wheel caster wheels is to form an axle receiving bore entirely through the wheel and attach a snap ring over an end of the axle projecting outwardly from each wheel. In this method the axle and snap ring are visible from the outside of the wheel and therefore require a hub cap or other decorative element to conceal the axle ends, which significantly increases the cost of the caster in this competitive market.
It is the primary object of the present invention to ameliorate the problems noted above in attaching wheels to relatively light-duty dual wheel casters.